1. Field of Invention
The inventive subject matter relates to a method of inducing an immune response against dengue virus using a prime-boost vaccination methodology.
2. Background
Dengue virus, the causative agent of dengue fever (DF) and dengue hemorrhagic fever (DHF), is a virus of the genus Flavivirus, a single-stranded enveloped RNA virus with positive polarity. Its RNA encodes approximately 3,400 amino acids. The virus exists as four antigenically-distinguishable serotypes.
Dengue fever is the most common human arbovirus infection worldwide and a serious public health concern accounting for estimates of 100 million infections annually (WHO 1986; Monath and Heinz 1996; Thomas, et al 2003). DF and DHF are found in most tropical areas including Africa, Asia, the Pacific, Australia, and the Americas.
Although the virus is capable of growing in a variety of species of mosquitoes, including Aedes albopictus, Aedes polynesiensis and Aedes scutellaris, Aedes aegypti is the most efficient mosquito vector because of its domestic habitat (Gubler 1988). Four antigenically distinct serotypes of dengue virus have been identified with all causing human diseases (Gubler, et al 1979; Henchal and Putnak 1990). Each of the four serotypes, although distinct, is similar enough to the others to elicit only partial cross-protection following infection (WHO 1986). Following infection, viremia is typically detected early at the onset of symptoms (Halstead 1997). Although many dengue infections are mild, some infections result in DHF and dengue shock syndrome (DSS), which are potentially fatal. This usually occurs in a small number of people during a second infection caused by a dengue virus that is different from the virus causing the first infection (Halstead 1997).
Dengue virus infection occurs following the bite of dengue virus-infected Aedes mosquitoes, which were previously infected by feeding on infected humans. Symptoms of dengue infection include high fever, severe headache, retro-orbital pain, development of a rash, nausea, joint and muscle pain, and usually start within five to six days following the bite of an infected mosquito. Symptoms of DHF also include marked sub-dermal bleeding, causing a purplish bruise, as well as bleeding from the nose, gums, and gastrointestinal (GI) tract. The fatality rate associated with DHF is at 6 to 30% with most deaths occurring in infants. The management of DHF is symptomatic and supportive, and is aimed at replacement of fluid loss (Nimmannitya 1996).
It is not possible to make an accurate diagnosis of mild or classic DF based on clinical features alone since many symptoms of DF resemble those of other diseases, such as chikungunya infection (Nimmannitya 1996), measles, influenza, and rickettsial infections. Differential diagnosis must include malaria and other viral, bacterial, and rickettsial diseases. Diagnostic methods for infection are typically based on detection of virus, viral antigens, genomic sequences, and detection of dengue-specific antibodies (Shu and Huang 2004). DHF can, in some cases, be more accurately diagnosed based on clinical signs and symptoms, including high continuous fever for 2 to 7 days, hepatomegaly, hemoconcentration, shock and thromocytopenia.
Most infections result in DF, which is self-limiting. However, DHF and DSS are life-threatening. Although vaccines against other flaviviruses, such as yellow fever and Japanese encephalitis, have been licensed, there are currently no efficacious vaccines to protect against DF, DHF or DSS.
Two dengue tetravalent live-attenuated vaccine (TLAV) candidates currently exist. However, both of these vaccines may be either reactogenic or poorly immunogenic in some recipients. Promising alternatives include chimeric viruses (e.g., Yellow fever/Dengue), recombinant proteins, inactivated viruses and nucleic acid (DNA) vaccines. The DNA vaccines may be particularly useful at eliciting a cell-mediated as well as a humoral immune response.
Experimental evidence suggests that, in non-human primates, dengue DNA vaccines, given alone, require several booster administrations and long intervals between the administrations in order to induce protective immunity. Other non-replicating vaccines such as the purified inactivated vaccines can often induce high titers of neutralizing antibody but these vaccines may be poor inducers of long-term immunological memory. Therefore, a safe, efficacious immunization method and composition is needed for the more timely induction of long-lasting immunity to dengue virus infection.